The Papova viruses, Polyoma and SV40, are oncogenic DNA viruses capable of both lytic infection and abortive infection resulting in cellular transformation. The murine cell line Balb/c 3T3 is susceptible to both types of infection by Papova viruses: lytic with polyoma and transformation with SV40. In any effort to understand the cellular functions required in these interactions, we have been isolating Balb/c 3T3 mutants which are temperature sensitive (ts) fro growth. Four, previously unreported, mutants have been isolated which show a decreased ability to synthesize DNA upon shift to the non-permissive temperature (38C). We are presently determining the nature of the defect and the susceptibility of these mutant cells to Papova virus infection. The role of a specific enzyme, a DNA endonuclease associated with purified SV40 virons, is being evaluated. We are presently comparing its properties with several other DNA endonucleases isolated from uninfected monkey cells and serum--likely non-viral sources of the virion enzyme. In addition, this enzyme has been purified from wild type SV40 and temperature- sensitive SV40 mutants.